A nitride semiconductor is characteristically a wide bandgap semiconductor. A wide bandgap semiconductor normally has high withstand voltage and low resistance. Therefore, attention is drawn to nitride semiconductors as devices for the next-generation power electronics. Typical devices for power electronics are FETs and diodes, and nitride semiconductors are being actively developed for FETs and diodes.
There is a type of nitride semiconductor that uses a hetero junction. When an AlGaN layer, an InAlN layer, or the like is formed on GaN, an electron channel called a two-dimensional electron gas is formed at the interface between the GaN and the AlGaN or InAlN. As the two-dimensional electron gas has high electron mobility, devices that have low resistance and are capable of high-speed operations can be produced. Accordingly, with nitride semiconductors, power devices having hetero structures are often formed.
Since only electrons that are majority carriers serve as carriers in a Schottky barrier diode that is a type of diode, such a diode excels in high-speed characteristics. It is said that the on-state voltage of a diode is generally low. A Schottky junction is formed by bringing a semiconductor into a Schottky metal, to form a Schottky barrier. In this manner, a rectifying operation is realized. The withstand voltage of a Si Schottky barrier diode is low. In a case where a Schottky barrier diode is manufactured with a wide-gap semiconductor, however, a device with a high withstand voltage can be formed because of its physicality. Still, the Schottky barrier of a wide-gap semiconductor is often higher than the Schottky barrier of Si, and the on-state voltage becomes as high as approximately 1 V, for example. A high on-state voltage implies that the conduction loss at the time of current flow in the diode is large. Therefore, in the case of a Schottky barrier diode, the on-state voltage is preferably low. In a case where a metal with a small work function is used as a Schottky metal to lower the on-state voltage, on the other hand, the leakage current increases, though the on-state voltage can be lowered.